An electronic tracking device attached to a mako shark in New Caledonia was found by a woman walking along a New Zealand beach, much to NIWA's delight.
The device was attached to the shark in May last year, and the shark looks to have swum into New Zealand waters within 60 days of it being attached.
The study is looking at how well sharks survive after being caught in commercial fisheries and then released again, to see whether one method of catch-and-release results in fewer deaths.
Clare Veltman was walking Te Araroa - a 3000km walking track between Cape Reinga to Bluff - when she came across the device in the sand.
"There are no shells along that part of the beach so when you do see an object it catches your eye," she said.
"I'm a wildlife scientist by trade, so I knew what it was and immediately thought about the person waiting to get the data it holds ... so I put it in my pack and decided to do something about it once I got to Kaitaia."
NIWA shark researcher Warrick Lyon said the tag holds on for about two months, then the attachment strap corrodes away and it floats to the surface and starts transmitting its data.
If the tag is actually recovered, a much higher level of detail can be taken from the device, he said.
Mr Lyon said there was evidence sharks travelled north from New Zealand, but little evidence showing they also return.
NIWA gave Ms Veltham a $200 reward for returning the device, which she used to buy a new sleeping mat for her walk.
It's believed there is another tag somewhere in the area of Lottin Point at Cape Runaway in the Bay of Plenty.
Mako sharks are the fastest travelling shark in the world, with a normal speed of about 40km/h and burst of up to 74km/h.
Females can reach 3.8m long and about 570kg in weight.